1. Field of the Disclosure
This invention relates to the synthesis of diiodoperfluoro-C3 to C7-alkanes I(CF2)nI, wherein n is 3 to 7 that produces little to no diiodoperfluoromethane (ICF2I) and diiodoperfluoroethane (I(CF2)2I).
2. Description of the Related Art
U.S. Pat. No. 6,002,055 discloses that dihaloperfluoroalkanes are useful as chain transfer agents for fluoroelastomers. This patent also discloses the synthesis of dihalodifluoromethanes and their homologues by reacting a fluorinated epoxide with dihalogen in the presence of catalyst, also referred to as metal promoter. Hexafluoropropylene oxide (HFPO) is the preferred epoxide and iodine appears to be the preferred halogen. Fifteen of the twenty-one Examples use iodine as the halogen. The catalyst is a zero-valent metal, examples of which are Ni and Cu or combinations of metals such as Ni/Cu, Ni/Zn, or a metal halide such as CuI. The catalyst is usually in the form of a metal slurry or may be the Ni alloy, such as Hastelloy® C, that lines the process vessel. The importance of the catalyst is expressed in terms of low yield when the catalyst is not present during the reaction, resulting in the yield of dihalodifluoromethanes and homologs, not being greater than about 30%. When excess HFPO is used as the reactant, it is disclosed that the major product is the higher homologues such as I(CF2)3I, diiodoperfluoropropane, as shown in Example 2. Example 2 discloses the reaction between 254 g of iodine with 500 g of HFPO, mol ratio (HFPO/iodine of 3.1 to 1, carried out in a Hastelloy® C autoclave at a temperature of 185° C. for 30 hours. In addition to substantial formation of CF3COF (perfluoroacetyl fluoride) (PAF) gas, the diodoperfluoroalkane formation in the reaction product is as follows:
GC Diiodoperfluoroalkanearea %ICF2I7.1ICF2CF2I1ICF2CF2CF2I58.7I(CF2)4I0.86I(CF2)5I0.33
A principal disadvantage of this result is the formation of relatively large amounts of ICF2I and ICF2CF2I accompanying the formation of the higher homologues. The compound ICF2CF2I is a chain termination agent when present in the polymerization process to make fluoroelastomers, and must be removed from the reaction product before the reaction product is used in the polymerization to make fluoroelastomer.
WO 2014/062450 discloses a process for making diiodoperfluoroalkanes of the formula I(CF2)nI where n=3 to 11, and where the amount of compounds of the formula I(CF2)nI, where n is 1 or 2, is small. The n=1 and n=2 compounds are disclosed to be toxic or hazardous. In addition, it is difficult to remove these compounds from the reaction product containing the higher homologues. '450 teaches the making of the diiodoperfluoroalkanes by reacting HFPO with iodine in the presence of a combination of metal catalysts, as follows: at least one of (a) a first metallic compound and a second metallic compound comprising molybdenum or (b) a metallic alloy that contains certain amounts of Ni and Mo. The first metallic compound preferably has a high surface area such as provided by a finely divided powder, but can be present as the liner of the reactor within which the reaction is carried out. Alternatively, the second metallic compound can be the metal lining of the reactor. Another alternative embodied in Examples 4-9 is the metal alloy being the reactor lining along with perforated Ni ribbon positioned within the reactor. The use of a molar ratio of HFPO to iodine of at least 3.2, at least 3.6, or even at least 3.9 is disclosed to provide selectivity in the reaction process to drive the reaction more towards the formation of the n=3 to 11 diiodoperfluoroalkanes, minimizing the formation of the n=1 and 2 compounds. The selection of the first and second metals or the metal alloy also contributes to this selectivity. The Examples, in which the reaction temperature is 170° C., disclose that when only Ni catalyst is present by addition to the reaction (Comparative Examples A and B), the amount of ICF2I and ICF2CF2I formed are relatively large as reported in Table 1. Tables 3 and 6 report the formation of smaller amounts of these compounds when a combination of metal catalyst is used. These tables also report the formation of large molecule compounds, e.g. I(CF2)8I, ICF2CF2CF(CF3)C(═O)F, and CF3CF(I)CF2CF2I. These compounds as well as the CF3CF(I)C(═O)F and CF3I are deleterious to the polymerization process to make fluoroelastomer and therefore require a removal step before the reaction product can be used in polymerization to make fluoroelastomer. The presence of the catalyst, at least one of which is not the reactor lining also requires a removal step, all of which adds to the cost of the catalytic process of '450.